gh-38409: `xgcd` for multiple arguments
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`xgcd` for more than 2 elements. Implement as an another option for the
existing `xgcd` function. If there's only one input `a` is given (and `b
= None` as default), then returns `xgcd` result for the tuple/list `a`.
It calls the 2-variable version of `xgcd` recursively. Also see
[this](https://ask.sagemath.org/question/73642/xgcd-for-several-
arguments/) post in the forum.
It would be great if anyone can suggest more complex testcases, e.g.
rings other than integers or polynomials.
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### :hourglass: Dependencies
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URL: https://github.com/sagemath/sage/pull/38409
Reported by: Seewoo Lee
Reviewer(s): grhkm21, Julian Rüth, Seewoo Lee
